See my recommended Macs for photographers and similar higher-end users.
Why would you go with 32GB memory from Apple, when it costs *less* to install 64GB?
MPG recommends buying the 8GB memory configuration of the 2019 iMac 5K, then upgrading the memory with 64GB of OWC memory, for a savings of about $550 (as this was written). Basically, you can get 64GB for less than 32GB from Apple. Plus you can get some money back for the existing modules it came with.
It’s an easy DIY install that takes only a few minutes: open the back panel, remove the existing modules, insert the new modules, reboot—see the install video.
Below, MPG recommends the 64GB of 4 X 16GB modules for the best price, until and unless you are confident of going to 128GB (not clear this is possible), in which two pairs of the 64GB as two 32GB modules (4 X 32GB) would be the ticket.